Typically in the detection of optical signals PIN diodes have been utilized in connection with a preamplifier. However, thermal noise of the preamplifier is a limiting factor on system performance. To overcome this limitation, the avalanche photodiode (APD) was developed. The internal gain mechanism of an avalanche photodiode provides gain prior to the preamplifier, such that performance of the system is then limited by the shot noise of the avalanche photodiode due to either the detected signal, background signals or the detector leakage current.
Although avalanche photodiodes have eliminated several of the problems associated with PIN diodes, a problem associated with the operation of avalanche photodiodes is the maintenance of an optimum gain. The gain of an avalanche photodiode is a nonlinear function of temperature. Not only can the gain vary, but if the gain is not controlled the avalanche photodiode can go into full avalanche breakdown which can cause catastrophic and irreversible damage to the avalanche photodiode itself. A further problem associated with avalanche photodiodes is that the noise from the avalanche photodiode is also an increasing function of gain. Therefore, it is desirable to operate the avalanche photodiode such that the gain is just sufficient to bring the shot noise amplified by the avalanche process to the level of the thermal noise of the preamplifier. Furthermore, an application problem associated with avalanche photodiodes is that as the background level varies, the optimum value of gain varies.
Prior systems have attempted to overcome these problems associated with avalanche photodiodes by using a technique in which a reference avalanche photodiode is used for thermal stabilization and the gain is set by an applied voltage such that the user predetermines the desirable gain. A further prior system provides a temperature compensation circuit that is matched to a particular avalanche photodiode. This circuit senses a temperature change and predicts the associated variation in gain. The bias voltage is then adjusted to a predetermined level to maintain a constant gain. A further biasing technique is described and claimed in U.S. Pat. No. 3,869,207 issued to Hermet et al on Mar. 4, 1975, and entitled "Laser Transmitter" in which the bias voltage on an avalanche photodiode is controlled by maintaining the avalanche photodiode noise level constant. Another technique for biasing an avalanche photodiode is described and claimed in U.S. Pat. No. 4,077,718 issued to Graham, Jr. et al on Mar. 7, 1978, and entitled "Receiver for Optical Radar".
Such prior art systems have required the system to be operating synchronously with a transmitted pulse which is subsequently received as a reflection from a target. Such systems operate on a monopulse basis only and are not capable of controlling the gain of an avalanche photodiode in the presence of a continuous wave signal. Furthermore, such systems do not operate to optimize the noise from the avalanche photodiode to that of the avalanche photodiode preamplifier to optimize the performance of the entire system. Additionally, such prior systems have included circuitry and techniques for providing thermal stablization to avalanche photodiodes in an attempt to prevent full breakdown of avalanche photodiodes.
A need has thus arisen for a gain control for an avalanche photodiode in which the avalanche photodiode and associated avalanche photodiode preamplifier performance are optimized. A need has further arisen for a gain control for an avalanche photodiode operable in the presence of monopulse and pulse signals such as a DC level, video signal, or other CW signals or other signals of unknown waveforms. Furthermore, a need has arisen for a gain control for an avalanche photodiode in which the system is capable of operating asynchronously with respect to transmitted pulses, the reflections of which are to be detected by the avalanche photodiode. Additionally, a need has arisen for a gain control for an avalanche photodiode system for the control of noise to eliminate the requirement for thermal stabilization circuitry associated with avalanche photodiodes.